Red Blood Cells
In the vascular system of an adult human being, blood has a volume of about 5 to 6 liters. Approximately one half of this volume is occupied by cells, including red blood cells (erythrocytes), white blood cells (leukocytes), and blood platelets. Red blood cells comprise the majority of the cellular components of blood. Plasma, the liquid portion of blood, is approximately 90 percent water and 10 percent various solutes. These solutes include plasma proteins, organic metabolites and waste products, and inorganic compounds.
The major function of red blood cells is to transport oxygen from the lungs to the tissues of the body, and transport carbon dioxide from the tissues to the lungs for removal. Very little oxygen is transported by the blood plasma because oxygen is only sparingly soluble in aqueous solutions. Most of the oxygen carried by the blood is transported by the hemoglobin of the erythrocytes. Erythrocytes in mammals do not contain nuclei, mitochondria or any other intracellular organelles, and they do not use oxygen in their own metabolism. Red blood cells contain about 35 percent by weight hemoglobin, which is responsible for binding and transporting oxygen.
Hemoglobin is a protein having a molecular weight of approximately 64,500 Daltons. It contains four polypeptide chains and four heme prosthetic groups in which iron atoms are bound in the ferrous state. Normal globin, the protein portion of the hemoglobin molecule, consists of two α chains and two β chains. Each of the four chains has a characteristic tertiary structure in which the chain is folded. The four-polypeptide chains fit together in an approximately tetrahedral arrangement, to constitute the characteristic quaternary structure of hemoglobin. There is one heme group bound to each polypeptide chain, which can reversibly bind one molecule of molecular oxygen. When hemoglobin combines with oxygen, oxyhemoglobin is formed. When oxygen is released, the oxyhemoglobin is reduced to deoxyhemoglobin.
Although the secondary and tertiary structure of various hemoglobin subunits are similar, reflecting extensive homology in amino acid composition, the variations in amino acid composition that do exist impart marked differences in hemoglobin's oxygen carrying properties. In addition, the quaternary structure of hemoglobin leads to physiologically important allosteric interactions between the subunits, a property lacking in monomeric myoglobin, which is otherwise very similar to the -subunit of hemoglobin.
Comparison of the oxygen-binding properties of myoglobin and hemoglobin illustrate the allosteric properties of hemoglobin that results from its quaternary structure and differentiate hemoglobin's oxygen-binding properties from that of myoglobin. The curve of oxygen-binding to hemoglobin is sigmoidal typical of allosteric proteins in which the substrate, in this case oxygen, is a positive homotropic effector. When oxygen binds to the first subunit, of deoxyhemoglobin, it increases the affinity of the remaining subunits for oxygen. As additional oxygen is bound to the second and third subunits, oxygen-binding is further, incrementally, strengthened, so that at the oxygen tension in lung alveoli, hemoglobin is fully saturated with oxygen. As oxyhemoglobin circulates to deoxygenated tissue, oxygen is incrementally unloaded and the affinity of hemoglobin for oxygen is reduced. Thus at the lowest oxygen tensions found in very active tissues the binding affinity of hemoglobin for oxygen is very low allowing maximal delivery of oxygen to the tissue. In contrast the oxygen-binding curve for myoglobin is hyperbolic in character indicating the absence of allosteric interactions in this process. When the affinity for oxygen is decreased, the sigmoidal curve is shifted to the right. This shift of the curve is commonly known as a “right shift”.
The allosteric oxygen-binding properties of hemoglobin arise directly from the interaction of oxygen with the iron atom of the heme prosthetic groups and the resultant effects of these interactions on the quaternary structure of the protein. When oxygen binds to an iron atom of deoxyhemoglobin it pulls the iron atom into the plane of the heme. Since the iron is also bound to histidine F8, this residue is also pulled toward the plane of the heme ring. The conformational change at histidine F8 is transmitted throughout the peptide backbone resulting in a significant change in tertiary structure of the entire subunit. Conformational changes at the subunit surface lead to a new set of binding interactions between adjacent subunits. The latter changes include disruption of salt bridges and formation of new hydrogen bonds and new hydrophobic interactions, all of which contribute to the new quaternary structure.
The latter changes in subunit interaction are transmitted, from the surface, to the heme binding pocket of a second deoxy subunit and result in easier access of oxygen to the iron atom of the second heme and thus a greater affinity of the hemoglobin molecule for a second oxygen molecule. The tertiary configuration of low affinity, deoxygenated hemoglobin (Hb) is known as the taut (T) state. Conversely, the quaternary structure of the fully oxygenated high affinity form of hemoglobin Hb(O2)4 is known as the relaxed (R) state.
Delivery of oxygen to tissues depends upon a number of factors including, but not limited to, the volume of blood flow, the number of red blood cells, the concentration of hemoglobin in the red blood cells, the oxygen affinity of the hemoglobin and, in certain species, on the molar ratio of intra-erythrocytic hemoglobins with high and low oxygen affinity. The oxygen affinity of hemoglobin depends on four factors as well, namely: (1) the partial pressure of oxygen, (2) the pH, (3) the concentration of the allosteric effector 2,3-diphosphoglycerate (DPG) in the hemoglobin, and (4) the concentration of carbon dioxide. In the lungs, at an oxygen partial pressure of 100 mm Hg, approximately 98% of circulating hemoglobin is saturated with oxygen. This represents the total oxygen transport capacity of the blood. When fully oxygenated, 100 ml of whole mammalian blood can carry about 21 ml of gaseous oxygen.
The effect of the partial pressure of oxygen and the pH on the ability of hemoglobin to bind oxygen is best illustrated by examination of the oxygen saturation curve of hemoglobin. An oxygen saturation curve plots the percentage of total oxygen-binding sites present in a unit of blood, or a sample that are occupied by oxygen molecules when solutions of the hemoglobin are in equilibrium with varying partial pressures of oxygen.
As stated above, the oxygen saturation curve for hemoglobin is sigmoid. Thus, binding the first molecule of oxygen increases the affinity of the remaining hemoglobin for binding additional oxygen molecules. As the partial pressure of oxygen is increased, a plateau is approached at which each of the hemoglobin molecules is saturated and contains the upper limit of four molecules of oxygen.
The reversible binding of oxygen by hemoglobin is accompanied by the release of protons, according to the equation:HHb++O2HbO2+kH+Thus, an increase in the pH will pull the equilibrium to the right and cause hemoglobin to bind more oxygen at a given partial pressure. A decrease in the pH will decrease the amount of oxygen bound.
In the lungs, the partial pressure of oxygen in the air spaces is approximately 90 to 100 mm Hg and the pH is also high relative to normal blood pH (up to 7.6). Therefore, hemoglobin will tend to become almost maximally saturated with oxygen in the lungs. At that pressure and pH, hemoglobin is approximately 98 percent saturated with oxygen. On the other hand, in the capillaries in the interior of the peripheral tissues, the partial pressure of oxygen is only about 25 to 40 mm Hg and the pH is also relatively low (about 7.2 to 7.3). Because muscle cells use oxygen at a high rate thereby lowering the local concentration of oxygen, the release of some of the bound oxygen to the tissue is favored. As the blood passes through the capillaries in the muscles, oxygen will be released from the nearly saturated hemoglobin in the red blood cells into the tissue. Hemoglobin will release about a third of its bound oxygen as it passes through the muscle capillaries, so that when it leaves the muscle, it will be only about 64 percent saturated. In general, the hemoglobin in the venous blood leaving the tissue cycles between about 65 and 97 percent saturation with oxygen in its repeated circuits between the lungs and the peripheral tissues. Thus, oxygen partial pressure and pH function together to affect the release of oxygen by hemoglobin
A third important factor in regulating the degree of oxygenation of hemoglobin is the allosteric effector 2,3-diphosphoglycerate (DPG). DPG is the normal physiological effector of hemoglobin in mammalian erythrocytes. DPG regulates the oxygen-binding affinity of hemoglobin in the red blood cells in relationship to the oxygen partial pressure in the lungs. In general, the higher the concentration of DPG in the cell, the lower the affinity of hemoglobin for oxygen.
When the delivery of oxygen to the tissues is chronically reduced, the concentration of DPG in the erythrocytes is increased in normal individuals. For example, at high altitudes the partial pressure of oxygen is significantly less. Correspondingly, the partial pressure of oxygen in the tissues is less. Within a few hours after a normal human subject moves to a higher altitude, the DPG level in the red blood cells increases, causing more DPG to be bound and the oxygen affinity of the hemoglobin to decrease. Increases in the DPG level of red blood cells also occur in patients suffering from hypoxia. This adjustment allows the hemoglobin to release its bound oxygen more readily to the tissues to compensate for the decreased oxygenation of hemoglobin in the lungs.
As normally isolated from blood, hemoglobin contains a considerable amount of DPG. When hemoglobin is “stripped” of its DPG, it shows a much higher affinity for oxygen. When DPG is increased, the oxygen-binding affinity of hemoglobin decreases. A physiologic allosteric effector such as DPG is therefore essential for the normal release of oxygen from hemoglobin in the tissues.
While DPG is the normal physiologic effector of hemoglobin in mammalian red blood cells, phosphorylated inositols are found to play a similar role in the erythrocytes of some birds and reptiles. Although IHP is unable to pass through the mammalian erythrocyte membrane, it is capable of combining with hemoglobin of mammalian red blood cells at the binding site of DPG to modify the allosteric conformation of hemoglobin, the effect of which is to reduce the affinity of hemoglobin for oxygen. For example, DPG can be replaced by inositol hexaphosphate (IHP), which is even more potent than DPG in reducing the oxygen affinity of hemoglobin. IHP has a 1000-fold higher affinity to hemoglobin than DPG (R. E. Benesch et al., Biochemistry, Vol. 16, pages 2594–2597 (1977)) and increases the P50 of hemoglobin up to values of 96.4 mm Hg at pH 7.4, and 37° C. (J. Biol. Chem., Vol. 250, pages 7093–7098 (1975)).
The oxygen release capacity of mammalian red blood cells can be enhanced by introducing certain allosteric effectors of hemoglobin into erythrocytes, thereby decreasing the affinity of hemoglobin for oxygen and improving the oxygen economy of the blood. This phenomenon suggests various medical applications for treating individuals who are experiencing lowered oxygenation of their tissues due to the inadequate function of their lungs or circulatory system.
Because of the potential medical benefits to be achieved from the use of these modified erythrocytes, various techniques have been developed in the prior art to enable the encapsulation of allosteric effectors of hemoglobin in erythrocytes. Accordingly, numerous devices have been designed to assist or simplify the encapsulation procedure. The encapsulation methods known in the art include osmotic pulse (swelling) and reconstitution of cells, controlled lysis and resealing, incorporation of liposomes, and electroporation. Current methods of electroporation make the procedure commercially impractical on a scale suitable for commercial use.
The following references describe the incorporation of polyphosphates into red blood cells by the interaction of liposomes loaded with IHP: Gersonde, et al., “Modification of the Oxygen Affinity of Intracellular Haemoglobin by Incorporation of Polyphosphates into Intact Red Blood Cells and Enhanced O2 Release in the Capillary System”, Biblthca. Haemat., No. 46, pp. 81–92 (1980); Gersonde, et al., “Enhancement of the O2 Release Capacity and of the Bohr-Effect of Human Red Blood Cells after Incorporation of Inositol Hexaphosphate by Fusion with Effector-Containing Lipid Vesicles”, Origins of Cooperative Binding of Hemoglobin, (1982); and Weiner, “Right Shifting of Hb-O2 Dissociation in Viable Red blood cells by Liposomal Technique,” Biology of the Cell, Vol. 47, (1983).
Additionally, U.S. Pat. Nos. 4,192,869, 4,321,259, and 4,473,563 to Nicolau et al. describe a method whereby fluid-charged lipid vesicles are fused with erythrocyte membranes, depositing their contents into the red blood cells. In this manner, it is possible to transport allosteric effectors such as inositol hexaphosphate into erythrocytes, where, due to its much higher binding constant, IHP replaces DPG at its binding site in hemoglobin.
In accordance with the liposome technique, IHP is dissolved in a phosphate buffer until the solution is saturated and a mixture of lipid vesicles is suspended in the solution. The suspension is then subjected to ultrasonic treatment or an injection process, and then centrifuged. The upper suspension contains small lipid vesicles containing IHP, which are then collected. Erythrocytes are added to the collected suspension and incubated, during which time the lipid vesicles containing IHP fuse with the cell membranes of the erythrocytes, thereby depositing their contents into the interior of the erythrocyte. The modified erythrocytes are then washed and added to plasma to complete the product.
The drawbacks associated with the liposomal technique include poor reproducibility of the IHP concentrations incorporated in the red blood cells and significant hemolysis of the red blood cells following treatment. Additionally, commercialization is not practical because the procedure is tedious and complicated.
In an attempt to solve the drawbacks associated with the liposomal technique, a method of lysing and the resealing red blood cells was developed. This method is described in the following publication: Nicolau, et al., “Incorporation of Allosteric Effectors of Hemoglobin in Red Blood Cells. Physiologic Effects,” Biblthca. Haemat., No. 51, pp. 92–107, (1985). Related U.S. Pat. Nos. 4,752,586 and 4,652,449 to Ropars et al. also describe a procedure of encapsulating substances having biological activity in human or animal erythrocytes by controlled lysis and resealing of the erythrocytes, which avoids the RBC-liposome interactions.
The technique is best characterized as a continuous flow dialysis system which functions in a manner similar to the osmotic pulse technique. Specifically, the primary compartment of at least one dialysis element is continuously supplied with an aqueous suspension of erythrocytes while the secondary compartment of the dialysis element contains an aqueous solution, which is hypotonic with respect to the erythrocyte suspension. The hypotonic solution causes the erythrocytes to lyse. The erythrocyte lysate is then contacted with the biologically active substance to be incorporated into the erythrocyte. To reseal the membranes of the erythrocytes, the osmotic and/or oncotic pressure of the erythrocyte lysate is increased and the suspension of resealed erythrocytes is recovered.
In related U.S. Pat. Nos. 4,874,690 and 5,043,261 to Goodrich et al., a related technique involving lyophilization and reconstitution of red blood cells is disclosed. As part of the process of reconstituting the red blood cells, the addition of various polyanions, including inositol hexaphosphate, is described. Treatment of the red blood cells according to the process disclosed results in a cell with unaffected activity. Presumably, the IHP is incorporated into the cell during the reconstitution process, thereby maintaining the activity of the hemoglobin.
In U.S. Pat. Nos. 4,478,824 and 4,931,276 to Franco et al., a second related method and apparatus is described for introducing effective agents, including inositol hexaphosphate, into mammalian red blood cells by effectively lysing and resealing the cells. The procedure is described as the “osmotic pulse technique.” In practicing the osmotic pulse technique, a supply of packed red blood cells is suspended and incubated in a solution containing a compound, which readily diffuses into and out of the cells, the concentration of the compound being sufficient to cause diffusion thereof into the cells so that the contents of the cells become hypertonic. Next, a trans-membrane ionic gradient is created by diluting the solution containing the hypertonic cells with an essentially isotonic aqueous medium in the presence of at least one desired agent to be introduced, thereby causing diffusion of water into the cells with a consequent swelling and an increase in permeability of the outer membranes of the cells. The increase in permeability of the membrane is maintained for a period of time sufficient only to permit transport of at least one agent into the cells and diffusion of the compound out of the cells. These fluxes must be coupled because polyanionic compounds do not simply diffuse across membranes but exchange for ions of equal charge.
Polyanions which may be used in practicing the osmotic pulse technique include pyrophosphate, tripolyphosphate, phosphorylated inositols, 2,3-diphosphoglycerate (DPG), adenosine triphosphate, heparin, and polycarboxylic acids which are water-soluble, and non-disruptive to the lipid outer bilayer membranes of red blood cells.
The osmotic pulse technique has several shortcomings including the fact that the technique is tedious, complicated and unsuited to automation. For these reasons, the osmotic pulse technique has had little commercial success.
Another method for encapsulating various biologically-active substances in erythrocytes is electroporation. Electroporation has been used for encapsulation of foreign molecules in different cell types including IHP red blood cells as described in Mouneimne, et al., “Stable rightward shifts of the oxyhemoglobin dissociation curve induced by encapsulation of inositol hexaphosphate in red blood cells using electroporation,” FEBS, Vol. 275, No. 1, 2, pp. 117–120 (1990).
The process of electroporation involves the formation of pores in the cell membranes, or in any vesicles, by the application of electric field pulses across a liquid cell suspension containing the cells or vesicles. During the poration process, cells are suspended in a liquid media and then subjected to an electric field pulse. The medium may be electrolyte, non-electrolyte, or a mixture of electrolytes and non-electrolytes. The strength of the electric field applied to the suspension and the length of the pulse (the time that the electric field is applied to a cell suspension) varies according to the cell type. To create a pore in a cell's outer membrane, the electric field must be applied for such a length of time and at such a voltage as to create a set potential across the cell membrane for a period of time long enough to create a pore.
Electroporation has been used effectively to incorporate allosteric effectors of hemoglobin in erythrocytes. In the article by Mouneime, et al., supra, it was reported that right shifts of the hemoglobin-oxygen dissociation curve in treated erythrocytes having incorporated IHP can be achieved. Measurements at 24 and 48 hours after loading with IHP showed a stable P50 value indicating that resealing of the erythrocytes was permanent. Furthermore, it was shown that red blood cells loaded with inositol hexaphosphate have a normal half life of eleven days. However, the results obtained by Mouneimne and his colleagues indicate that approximately 20% of the re-transfused cells were lost within the first 24 hours of transfusion. U.S. Pat. Nos. 5,720,921, 6,090,617, and 6,074,605 are incorporated herein by reference in their entirety and all disclose an apparatus and method for flow electroporation.
The electroporation methods disclosed in the prior art are not suitable for processing large volumes of sample, nor use of a high or repetitive electric charge. In addition, the stability of the P50 right shift as well as the stability of the red blood cells has not proved adequate for clinical use. Furthermore, the methods are not suitable for use in a continuous or “flow” electroporation chamber. Available electroporation chambers are designed for static use only; namely, processing of samples by batch. Continuous use of a “static” chamber results in over heating of the chamber and increased cell lysis. Furthermore, the existing technology is unable to incorporate a sufficient quantity of IHP in a sufficient percentage of the cells being processed to dramatically change the oxygen carrying capacity of the blood. In addition, the prior art methods require elaborate equipment and are not suited for loading red blood cells of a patient at the point of care. Thus, the procedure is time consuming and not suitable for use on a commercial scale.
What is needed is a simple, efficient and rapid method for encapsulating biologically-active substances in erythrocytes in sufficient volume while preserving the integrity and biologic function of the cells. The potential therapeutic applications of biologically altered blood cells suggests the need for simpler, and more effective and complete methods of encapsulation of biologically-active substances, including allosteric effectors of hemoglobin in intact erythrocytes.
There are numerous clinical conditions that would benefit from treatments that would increase of oxygen bound to hemoglobin. For example to tissue, the leading cause of death in the United States today is cardiovascular disease. The acute symptoms and pathology of many cardiovascular diseases, including congestive heart failure, ischemia, myocardial infarction, stroke, intermittent claudication, and sickle cell anemia, result from an insufficient supply of oxygen in fluids that bathe the tissues. Likewise, the acute loss of blood following hemorrhage, traumatic injury, or surgery results in decreased oxygen supply to vital organs. Without oxygen, tissues at sites distal to the heart, and even the heart itself, cannot produce enough energy to sustain their normal functions. The result of oxygen deprivation is tissue death and organ failure. Another area that would benefit from treatments that would increase delivery of oxygen bound to hemoglobin to tissue is racing animals, athletes, etc.
Another area is in treating diseases such as adult respiratory distress syndrome because administration of blood that is capable of increased delivery of oxygen to the peripheral tissues will ease the pressure of loading hemoglobin in the lungs.
Although the attention of the American public has long been focused on the preventive measures required to alleviate heart disease, such as exercise, appropriate dietary habits, and moderation in alcohol consumption, deaths continue to occur at an alarming rate. One approach to alleviate the life-threatening consequences of cardiovascular disease is to increase oxygenation of tissues during acute stress. The same approach is also appropriate for persons suffering from blood loss or chronic hypoxic disorders, such as congestive heart failure.
Another condition that could benefit from an increase in the delivery of oxygen to the tissues is anemia. A significant portion of hospital patients experience anemia or a low “crit” caused by an insufficient quantity of red blood cells or hemoglobin in their blood. This leads to inadequate oxygenation of their tissues and subsequent complications. Typically, a physician believes that he or she can temporarily correct this condition by transfusing the patient with units of packed red blood cells.
Enhanced tissue oxygenation may also reduce the number of heterologous transfusions and allow use of autologous transfusions in more cases. The current method for treatment of anemia or replacement of blood loss is transfusion of whole human blood. It is estimated that three to four million patients receive transfusions in the U.S. each year for surgical or medical needs. In situations where there is more time or where the religious beliefs of the patient forbid the use of heterologous blood for transfusions, it is advantageous to completely avoid the use of donor or heterologous blood and instead use autologous blood.
Often the amount of blood that can be drawn and stored prior to surgery limits the use of autologous blood. Typically, a surgical patient does not have enough time to donate a sufficient quantity of blood prior to surgery. A surgeon would like to have several units of blood available. As each unit requires a period of several weeks between donations and can not be done less than two weeks prior to surgery, it is often impossible to sequester an adequate supply of blood. By processing autologous blood with IHP, less blood is required and it becomes possible to completely avoid the transfusion of heterologous blood.
As IHP-treated red blood cells transport 2–3 times as much oxygen as untreated red blood cells, in many cases, a physician will need to transfuse fewer units of IHP-treaded red blood cells. This exposes the patient to less heterologous blood, decreases the extent of exposure to viral diseases from blood donors and minimizes immune function disturbances secondary to transfusions. The ability to infuse more efficient red blood cells is also advantageous when the patient's blood volume is excessive. In other more severe cases, where oxygen transport is failing, the ability to rapidly improve a patient's tissue oxygenation is life saving.
Although it is evident that methods of enhancing oxygen delivery to tissues have potential medical applications, currently there are no methods clinically available for increasing tissue delivery of oxygen bound to hemoglobin. Transient elevations of oxygen deposition (6 to 12 hours) have been described in experimental animals using either DPG or molecules that are precursors of DPG. The natural regulation of DPG synthesis in vivo and its relatively short biological half-life, in addition to its lower efficiency in modulating Hb properties, however, limit the DPG concentration and the duration of increased tissue PO2, and thus limit its therapeutic usefulness.
What is needed is a simple, efficient and rapid method for encapsulating biologically-active substances, such as IHP, in erythrocytes without damaging the erythrocytes beyond their ability to produce a clinical effect. An important requirement for any system of introducing IHP into red blood cells is that the right shift of the sigmoidal oxygen-binding curve be substantially stable and the red blood cell must be substantially similar to untreated red blood cells.
Gene Transfection
Efforts to develop human gene therapies have their roots in the 1950s, when early successes with kidney transplantation led to speculation that it might be possible to transplant cells from a normal individual into a patient suffering from a genetic disease. Soon after the discovery of the enzymatic defects in Gaucher's and Niemann-Pick disease, scientists considered organ and bone marrow transplantation and enzyme supplementation to treat rare genetic disorders (Brady, R., NEJM 275:312 (1966)). By the late 1960s and early 1970s, several investigators speculated that it also might be possible to introduce genes into a patient's own cells, and the cloning of the first human genes only a few years later intensified work in the field.
Until recently, almost all of the theoretical and experimental work on human gene therapy was centered on extremely rare genetic diseases, and gene therapy has come to mean, to many in the field, the modification of a patient's genes to treat a genetic disease. However, gene therapy has far wider applications than simply treatment of a genetic disease. Gene therapy is perhaps more appropriately described as medical intervention in which cells, either from the individual to be treated or another appropriate source, are modified genetically to treat or cure any condition, regardless of etiology, that will be ameliorated by the long-term delivery of a therapeutic protein. Gene therapy can therefore be thought of as an in vivo protein production and delivery system, and almost all diseases that are currently treated by the administration of proteins are candidates for treatment using gene therapy.
Gene therapy can be divided into two areas: germ cell and somatic cell gene therapy. Germ cell gene therapy refers to the modification of sperm cells, egg cells, zygotes, or early stage embryos. On the basis of both ethical and practical criteria, germ cell gene therapy is inappropriate for human use. In contrast to germ cell gene therapy, somatic cell gene therapy would affect only the person under treatment (somatic cells are cells that are not capable of developing into whole individuals and include all of the body's cells with the exception of the germ cells). As such, somatic cell gene therapy is a reasonable approach to the treatment and cure of certain disorders in human beings.
In conventional somatic cell gene therapy system, somatic cells (e.g., fibroblasts, hepatocytes, or endothelial cells) are removed from the patient, cultured in vitro, transfected with the gene(s) of therapeutic interest, characterized, and reintroduced into the patient. The means by which these five steps are carried out are the distinguishing features of a given gene therapy system.
Presently-available approaches to gene therapy include the use of infectious vectors, such as retroviral vectors, which include the genetic material to be expressed. Such approaches have limitations, such as the potential of generating replication-competent viruses during vector production; recombination between the therapeutic virus and endogenous retroviral genomes, potentially generating infectious agents with novel cell specificities, host ranges, or increased virulence and cytotoxicity; independent integration into large numbers of cells, increasing the risk of a tumorigenic insertional event; limited cloning capacity in the retrovirus (which restricts therapeutic applicability) and short-lived in vivo expression of the product of interest. A better approach to providing gene products, particularly one that avoids the risks associated with presently available methods and provides long-term production, would be valuable.
A method for encapsulating genetic material in various cell populations is electroporation. Electroporation has been used for encapsulation of foreign molecules in different cell types including IHP red blood cells as described in Mouneimne, et al supra. The electroporation methods disclosed in the prior art are not suitable for processing large volumes of sample, nor use of a high or repetitive electric charge. One of the problems with electroporation of cells has always been the excess heat that occurs during the electroporation process. This heat generated by the electroporation process causes extensive damage to living cells. In addition, the stability of the transformed cells has not proved adequate for clinical use. Furthermore, the methods are not suitable for use in a continuous or “flow” electroporation chamber. Most available electroporation chambers are designed for static use only. Namely, processing of samples by batch. Continuous use of a “static” chamber results in over heating of the chamber and increased cell lysis. Furthermore, the existing technology is unable to incorporate a sufficient quantity of genetic material in a sufficient percentage of the cells being processed. In addition, the prior art methods require elaborate equipment and are not suited for transforming cells of a patient at the point of care. Thus, the procedure is time consuming and not suitable for use on a commercial scale.
Prior Art Flow Cells
U.S. Pat. No. 5,676,646 discloses a flow electroporation apparatus with a flow cell comprising two electrodes separated by a non-conductive spacer, the spacer defining a flow path. The major problem with this flow cell as well as other prior art flow cells is that the surface area of the electrode is not sufficient to dissipate heat as the cells are being electroporated. Thus, the heat buildup in the prior art flow cells is very large as the cells are being electroporated. This heat build up can cause damage to cells and cell components and decrease the efficiency of the electroporation process.
What is needed is a simple, efficient and rapid method for encapsulating genetic material in cells in sufficient volume in real-time, while preserving the integrity and biologic function of the cells. The potential therapeutic applications of transformed cells suggests the need for simpler, and more effective and complete methods of encapsulation of genetic material in intact cells. In addition, a flow cell is needed that is capable of removing heat so that damage to living cells that are being electroporated is kept to a minimum.